Onomatopoeia:
* "AAAAAAA!"
* "EEEEEEEK!"
* "OH NO!"
* "HELP!"
* "SHOUTING"
Descriptive words:
* "A bloodcurdling scream"
* "A piercing shriek"
* "A deafening roar"
* "A desperate cry"
* "A chilling wail"
Ultimately, the best way to "spell out" a scream is to imagine the sound itself.